Lithographic printing is the process of printing from specially prepared surfaces, some areas of which are capable of accepting ink (oleophilic areas) whereas other areas will not accept ink (oleophobic areas). The oleophilic areas form the printing areas while the oleophobic areas form the background areas.
Two basic types of lithographic printing plates are known. According to a first type, so called wet printing plates, both water or an aqueous dampening liquid and ink are applied to the plate surface that contains hydrophilic and hydrophobic areas. The hydrophilic areas will be soaked with water or the dampening liquid and are thereby rendered oleophobic while the hydrophobic areas will accept the ink. A second type of lithographic printing plates operates without the use of a dampening liquid and are called driographic printing plates. This type of printing plate comprises highly ink repellant areas and oleophilic areas. Generally the highly ink repellant areas are formed by a silicon layer.
Lithographic printing plates can be prepared using a photosensitive lithographic printing plate precursor, also called imaging element. Such imaging element is exposed in accordance with the image data and is generally developed thereafter so that a differentiation results in ink accepting properties between the exposed and unexposed areas.
Examples of photosensitive lithographic printing plate precursors are for example the silver salt diffusion transfer (hereinafter DTR) materials disclosed in EP-A-410 500, EP-A-483 415, EP-A-423 399, imaging elements having a photosensitive layer containing diazonium salts or a diazo resin as described in e.g. EP-A-450 199, imaging elements having a photosensitive layer containing a photopolymerizable composition as described in e.g. EP-A-502 562, EP-A-491 457, EP-A-503 602, EP-A-471 483 or DE-A-4 102 173.
Alternatively a lithographic printing plate may be prepared from a heat mode recording material as a lithographic printing plate precursor. Upon application of a heat pattern in accordance with image data and optional development the surface of such heat mode recording material may be differentiated in ink accepting and ink repellant areas. The heat pattern may be caused by a direct heating source such as a thermal head but may also be caused by a light source as e.g. a laser. In the latter case the heat mode recording material will include a substance capable of converting the light into heat. Heat mode recording materials that can be used for making a lithographic printing plate precursor are described in e.g. EP-A-685 333, DE-A-2 512 038, FR-A-1 473 751, Research Disclosure 19201 of April 1980 or Research Disclosure 33303 of January 1992.
As support for the above mentioned lithographic printing plates there are known metal support such as e.g. aluminum and flexible supports such as e.g. paper or polyester film supports. Generally the flexible supports are used for short run jobs where they have a cost advantage over metal supports. Furthermore, if a transparent flexible support is used exposure of the lithographic printing plate precursor may proceed through the support which allows the use of cameras without special optics.
A disadvantage of the use of flexible supported lithographic printing plates is however the inherently low dimensional stability which is one cause which limits the printing endurance of the printing plate especially when two or more images need to be printed over each other in register such as e.g. in color printing. Attempts have therefore been made to increase the dimensional stability of a flexible supported printing plate by e.g. using a thicker flexible support. Although this brings some improvement, there is still a need for further improvement. Furthermore, the use of a thicker support may cause problems in some processing equipment used for processing the printing plate precursor. Printing with plates with a flexible support happens mostly on low quality presses. However, on these presses is often plate slippage (the slip out of the clamp) the limiting factor. In most cases this is due to a limitation of the clamping system. Mostly these presses are equipped for the processing of paper plates and less suited for plates with a plastic base. Because flexible, non-paper-based printing plates have many advantages, more and more printing plates with a plastic base are used.
In JP-236 692/63 (Kokai) it has been proposed to adhere the flexible support of the printing plate to a metallic plate using a hot-melt adhesive. This method is cumbersome and does no longer present a cost advantage over the use of a lithographic printing plate precursor having a metal support.
EP-A-644 064 discloses a lithographic printing plate precursor comprising on a first side of a flexible support a surface capable of being differentiated in oleophilic and oleophobic areas upon imaging and on a second side opposite to said first side a layer comprising micro-particles of pressure-sensitive adhesive covered with a protective stripping layer. The production of said material is a cumbersome and costly process.